Their study concentrated on the McMurdo dry valleys, the largest ice-free area on the continent. It is a cold desert, with the largest animals soil invertebrates.

Of these the most widely distributed are soil nematodes, a sort of worm.

The team, led by Dr Peter Doran of the Department of Earth Sciences, University of Illinois at Chicago, found that the valleys had cooled by 0.7 degrees C per decade between 1986 and 2000, with pronounced summer and autumn trends.

They say: "We believe that climate cooling has significantly impacted ecosystem properties in the valleys."

They believe the 9% fall in primary production which they recorded in one of the valley lakes could produce a system so depleted in stored organic carbon that it might act as a source of CO2.

The researchers note: "Although other studies have cited a trend of continental warming in Antarctica, the trends are sensitive to the period analysed and to the distribution of [weather] stations.

Wider impact expected

"The large-scale cooling reported here results from an approach designed to avoid over-weighting of station-dense regions (for example, the peninsula) in the evaluation of overall trends.

"We propose that prolonged summer cooling will diminish aquatic and soil biological assemblages throughout the valleys, and possibly in other terrestrial Antarctic ecosystems.

"Summer temperatures are the critical driver of Antarctic terrestrial ecosystems, and our data are the first, to our knowledge, to highlight the cascade of ecological consequences that result from the recent summer cooling."

The Intergovernmental Panel on Climate Change (IPCC) says the Antarctic peninsula is "very vulnerable to projected climate change and its impacts".

Hard to predict

It adds: "The interior of Antarctica is less vulnerable, because the temperature changes envisaged over the next century are likely to have little impact and very few people are involved.

"However, there are considerable uncertainties about the mass balance of the Antarctic ice sheets and the future behaviour of the West Antarctic ice sheet (which has a low probability of disintegration over the next century).

"Changes in either could affect sea level and southern hemisphere climates."